The advancement of optical devices greatly promotes the development of optical communications. The rapid advancement of Wavelength Division Multiplexing (WDM) devices and optical amplifiers has made it possible to transmit data at a rate of Tbps through a single optical fiber. As the optical transmission technologies (e.g., coding, modulation, etc.) become matured, long-range and super long-range transmissions have emerged. Up to now, commercial transmission without electric regenerator can achieve transmission of more than 2000˜4000 Km. With the increasing of the bearing capacity of a single optical fiber and the extending of transmission range of transmission without electric regenerator, an all-optical transmission network (OTN) has been put on the agenda.
Owing to the transparency of optical signals, the all-optical transmission network has greatly reduced the cost and brings no similar “electronic bottleneck” effect that occurs in electric signal processing, and the processing capacity of the all-optical transmission network is not restricted by signal rate, protocol, or format, etc. Therefore, the all-optical transmission network is an ideal communication network. As technologies of reconfigurable devices, such as Wavelength Blocker (WB) and Wavelength Select Switch (WSS), become matured, it is practically possible to construct optical transmission networks on the basis of all-optical signal processing.
Due to the transparency of optical signals, no channel-associated overhead information can be carried, and therefore it is unable to monitor the quality of optical signals in the whole course and thus unable to take appropriate measures according to the quality of the optical signals; and it is unable to detect the sources and destinations of the optical signals, avoid possible wrong associations in optical signals, reroute the optical signals, or perform any other processing on the optical signals according to the channel-associated overhead of optical signals. As a result, it is difficult to construct a telecommunication-level optical transmission network that can be comparable to electric signal transmission networks.
International Telecommunication Union-Standardization Sector (ITU-T) proposes in G.709 an out-of-band control method, in which an out-of-band optical supervisory channel (OSC) is utilized to transmit information of different optical channels, thereby implementing management and maintenance of different optical signals. However, the out-of-band OSC can only carry out routing and protective switching over optical signals, but cannot realize supervisory control over quality or performance of optical signals, and therefore it is unable to determine whether the quality of optical signals meets requirements. The out-of-band OSC only transmits part of overheads and it is unable to determine whether the rerouting of optical signals by the reconfigurable devices actually accomplishes channel selection actions specified for the optical signals; and in addition, the failure of the out-of-band OSC itself will also trigger processing on optical signals; although reliability of the out-of-band OSC can be increased by means of a backup strategy, that will bring additional cost and complexity.
In U.S. Pat. No. 5,513,029 entitled “Method and Apparatus for Monitoring Performance of Optical Transmission System”, a method for monitoring quality of optical signals is disclosed, in which, by adding a low-frequency interference signal to the laser driving signal of each wavelength conversion unit and modulating the low-frequency interference signal to the output signal of the laser, an identification signal corresponding to the low-frequency interference signal is loaded onto a wavelength signal to form an optical signal. Since the modulation depth of the low-frequency interference signal is usually less than 10%, it has a relatively little effect on the main optical channel. At a monitoring point, the low-frequency interference signal is filtered off from the optical signal during the receiving of the optical signal by a low-frequency receiver. Since the component ratio between the low-frequency interference signal and the wavelength signal is constant, the power of the wavelength signal can be detected by detecting the power of the corresponding low-frequency interference signal; and information, such as optical signal-noise ratio (OSNR), on the corresponding wavelength signal can be calculated by measuring the overall optical power. Although this method can realize monitoring of the quality of the wavelength signal, it has some disadvantages as follows: 1. Different wavelengths must be identified with different frequencies, and the frequency interval must be greater than the frequency resolution of the monitoring point, as a result, the available identification signals are very limited; 2. Although the low-frequency interference signal has a relatively little effect on the main optical channel, it still has some effect on the quality of the wavelength signal; 3. The corresponding relationship between the wavelength signal and the low-frequency interference signal must be configured in advance; otherwise it will be difficult to know which channel of the wavelength signal is monitored; 4. In the method, the optical signals cannot carry channel-associated overhead; protective switching, other channel selection commands and other indication information must be settled through other methods.
In US patent applications No. US20030067646, US20030067647, and US20030067651, all entitled “Channel Identification in Communications Networks”, a new method for monitoring quality of optical signals is disclosed, in which two or more frequencies are utilized to identify a same wavelength signal, the wavelength signal has only one frequency identifier at the same time. At a monitoring point, by detecting frequency identifier and time dependence, it can be judged whether the wavelength signal exists; and the corresponding wavelength power and optical signal quality (e.g., OSNR) can be deduced from the time-averaged power. Since in the method more than one frequency is utilized to identify a same wavelength signal, in the case that the same number of available frequency points are used, the method can realize more identification functions than the previous prior art. However, that method still has the following disadvantages: 1. The energy of the low-frequency signal is relatively concentrated, and still has some effect on the wavelength signal; 2. The corresponding relationship between frequency identifier and wavelength still has to be configured in advance, in order to ascertain to which channel the wavelength signal detected by the monitoring point belongs; 3. The optical signals cannot carry channel-associated overhead information; and therefore protective switching, channel selection commands and other indication information have to be settled through other methods.
In the Chinese patent application No. CN98804006.9 entitled “Method for Transmitting Additional Data Signals and Useful Data Signals through Optical Connection”, another method is disclosed, in which a spread spectrum signal is formed by performing a spectrum spreading operation on an additional data signal, and then is superposed at a low amplitude with a useful data signal; after the data signal is received at the receiving end, the spread spectrum signal is separated from the useful data signal by means of frequency domain separation, and then the spread spectrum data is processed correspondingly to recover the original additional data signal. The method solves the problem of the transmission of channel-associated overhead; however, it does not solve the problems of optical signal quality monitoring and differentiation among different identifiers.
In an all-optical network, it is required that additional channel-associated overhead information is added at different transmitting points and is extracted at subsequent monitoring points. The overhead information is required to accomplish monitoring of quality of optical signals and carry channel-associated signaling, such as Automatic Protect Switch (APS) signaling for protective switching, Open Shortest Path First (OSPF) signaling for rerouting, and other information (e.g., source node ID and destination node ID, which are used to identify the source and termination of optical signals). None of the above prior arts meets the above requirements.